Inhibition of cholesterol oxidase in a patient with Cushing's syndrome

Serum from a patient with Cushing's syndrome who was being treated with mitotane contained components that interfered with determination of cholesterol in the Du Pont aca. A measured concentration of cholesterol of 4.19 g/L in the undiluted serum increased to a calculated concentration of 9.50 g/L in diluted serum. Adding additional cholesterol oxidase (EC 1.1.3.6) overcame the reaction inhibition in the undiluted sample; adding additional cholesterol esterase (EC 3.1.1.13) had no effect. There is the potential for clinically significant underestimation of cholesterol with the aca in such patients, because the interference may remain undetected. On the other hand, the aca accurately quantified undiluted specimens containing as much as 10 g of cholesterol per liter when mitotane was not present.